Generator armature test



April 15, 1952 F. J. FQUST ET AL GENERATOR ARMATURE TEST 9 Sheets-Sheet 1 Filed Oct. 7, 1948 PA 1 4.11.41. U m .5 H 7% 15L J J. Q5 1 ll. n w L W //v vE/v Tcws FLOYD J. F005 7 ROBE/QT A. THOMPSON THE/R HTTORWEYJ April 15, 1952 F. J. FOUST ET AL 2,593,131

GENERATOR ARMATURE TEST Filed Oct. 7, 1948 9 Sheets-Sheet 2 //vv/v 70 35 a 4 FL 0Y0 J. Fol/5r, 01 vr/v w CH/LDPESS JP, 5. DWIGHT a (PM, 905m 74. mow- 5mm April 15, 1952 F. J. FOUST ET AL 2,593,131-

GENERATOR ARMATURE TEST Filed Oct. '7, 1948 9 Sheets-Sheet 5 VEN TOPS FLOYD J. FOUST 0L VE/V l V. CH/LDRESS JP.

THE/P KITTOQNEYS Apri 1 2 F. J. FOUST ET AL GENERATOR ARMATURE TEST Filed Oct. 7, 1948 9 Sheets-Sheet 4 A ril 15, 1952 F. J. FOUST ET AL 2,593,131

GENERATOR ARMATURE TEST Filed Oct. 7, 1948 9 Sheets-Sheet 5 467;? F7 TTOP/VEYS April 15, 1952 F. J. ous'r ETAL 2,593,13'1

GENERATOR ARMATURE TEST Filed Oct. 7, 1948 9 Sheets-Sheet 7 M/VE/VTO/7S FLOYD J F0057 OLVE/VW. (H/[DRESS JP. DW/GHT 0 C P/M ROBERTA. THOMPSON Ap 1952 F. J. FOUST ETAL 2,593,131

GENERATOR ARMATURE TEST Filed 001;. 7, 1948 9 Sheets-Sheet 8 M #T I 21 7 INVENTORJ' 202 r FLOYD J. FOUST P055 TA THOMPSON mw THE/I? d T TORN EYS A nl 15, 1952 F. J. FOUST ET AL 2,593,131

GENERATOR ARMATURE TEST Filed Oct. 7, 1948 9 Sheets-Sheet 9 [///a// Flam/May rEsr--- ll illllllll I10 I I $0 240 2 800 w 0 POL LEPJ M6465 6170/70 7237' C 0/7/70 774 TOR THE/l? flTTORNEYS Patented Apr. 15, 1952 GENERATOR ARMATURE 'rns'r Floyd J. Foust, Olven W. Childress, Jr., Dwight 0. Crim, and Robert A. Thompson, Anderson, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application October 7, 1948, Serial No. 53,362

3 Claims. 1

This invention relates to the testing of armatures of dynamo electric machines and its object is to provide apparatus for testing the armature for grounded circuits, open circuits and short circuits. In the disclosed embodiment thereof, the invention provides mechanical means with which a dynamo armature can be easily coupled for effecting its rotation during the test and contactors engageable with its commutators and connected with electrical circuits which operate successively first, to burn out small metallic particles making a connection between the commutator bars and ground, second, to test for a permanent ground, and third, to subject the armature to a high frequency test during which the armature coils are connected with a resonant circuit inductively related to an oscillator. The high frequency test circuit includes a meter which will indicate whether the armature is good or whether any of its coils are short-circuited or open-circuited or grounded.

Further objects and advantages of the present invention will. be apparent from the following description, reference being had to the accompanying drawings, wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

Figs. 1, 2 and 3 are front, side and top views, respectively, of the cabinet enclosing the testing apparatus and supporting a panel with a meter and indicator lamps mounted thereon.

Figs. 4 and 5 together form a front elevation of a mechanism which receives and rotates the armature during the test, certain switches (shown in Fig. 6) being omitted for sake of clearness.

Fig. 6 is a side view in the direction of arrow 6 of Fig. 5 and includes the switches omitted in Fig. 4.

Fig. 7 is a fragmentary view on line l-l of Fig. 5.

Fig. 8 is a fragmentary top view in the direction of arrow 8 of Fig. 7.

Fig. 9 is a side view of an adapter which can be used in place of the one shown in Fig. '7.

Fig. 10 is a sectional view on line Ill-l0 of- 2 addition of a resilient band 45 and detent balls 46.

Fig. 14 is an end view of another adapter; Fig. 15 is a sectional view thereof on line l5- l5 of Fig. 14.

Fig. 16 is an enlarged sectional View on line I6l6 of Fig. 14.

Figs. 17, 18, 19, 20 and 21 are sectional views taken, respectively, on lines |'|--|1, l8|8, I9l9, 20-40 and 2l--2l of Fig. 4.

Fig. 22 is a view in the direction of arrow 22 of Fig. 4.

Fig. 23 is a sectional view on line 2323 of Fig. 22.

Fig. 24 is a fragmentary sectional view on line 2424 of Fig. 21.

Fig. 25 is a plan view of a commutator contact roller supporting unit.

Fig. 26 is a view similar to Fig. 25 with the top plate of said unit removed.

Fig. 27 is a sectional view on line 21-21 of Fig. 26.

Fig. 27A is a sectional view on line TIA-21A of Fig. 26.

Fig. 28 is a bottom view of the unit.

Fig. 29 is an end view in the direction of arrow 29 of Fig. 28.

Fig. 30 is a sectional view on line 30-30 of Fig. 29.

Fig. 31 is a view of a group of parts of the roller supporting unit.

Fig. 32 is a wiring diagram of electrical apparatus including the testing circuits.

Fig. 12 is a view in the direction ofarrow l2 v,

33 is a portion of the diagram of Fig. 32 showing its connections with commutator segments.

Fig. 34 is a diagram of a resonant curve.

Fig. 35 is a diagram showing meter readings under different conditions.

Fig. 36 is a timing chart.

Referring to Figs. 1, 2 and 3, the apparatus comprises a cabinet IO containing apparatus boxes I 1, i2 and I3 for housing relay switches, an oscillator and an amplifier and a power supply respectively. The top 14 of the table supports a plate [5 which supports certain mechanical parts. The top also supports a bracket l6 from which is suspended a panel P supporting signal lamps Ll, L2 and L3, a meter M and a resistance control RC.

Referring to Figs. 4, 5 and 6, plate l5 supports a plate I! which supports an electric motor 20 z 4) which drives, through pulley 2I, belt 22 an ulley 23, speed reducing gearing contained in afhousing 24 and connected by couplin'g'members and an intermediate disc 23 with a shaft 21 which is journaled in a bearing 29 supported by a bracket 28. Shaft 21 is connected with a bevel gear 30 meshing with a bevel gear 3| with a shaft 32 (Fig. 7) journaled in bearing 33 supported by a bracket 34 supported by plate Il. Shaft 32 has a flange 35 which receives one end of a tubular shaft 36 secured to the shaft 32 by a screw 31. Shaft 36 provides a recess 33 and a pin 39 extending into the recess. Recess 33 is adapted to receive one of a series of adapters it having central bores M of different diameters which receive armature shafts. The adapt-er 45, as shown in Figs. 9 and 10, has a shank 4! provided with a bayonet slot 42 cooperating with the pin 39 to secure the adapter to the shaft 35. The adapter has a bore 311 for receiving an end portion of the armature shaft and a bore 43b of larger diameter for receiving a portion of the armature shaft of larger diameter. adapter 40 has a groove 42 for receiving an elastic band 45 which urges inwardly a plurality of balls 46 guided by transverse holes 41 (Fig. 16). Inward movement of the balls 46 is limited by their engagement with the conical surface 48 of the adapter. The function of the balls 56 is to engage a groove or key-way slot in the armature shaft and thus provide driving engagement between it and the shaft 35. The cross slots 42 in the exposed end of the adapter to will receive a tool such as a wide-bit screw driver used to tighten the bayonet slot connection between the adapter 43 and the shaft 36. Instead of using a shaft 35 with an adapter, the shaft 59 (Figs. 14

and 15) having an integral adapter can be substi- 1 tuted. The parts of shaft and parts associated therewith which correspond to those of the adapter 69 are designated by the same numbers with primes affixed.

Plate I5 (Figs. '7 and 8) supports a ring 5I having a bore 52 which clears the armature commutator 60 and a bore 53 which clears the wires leading from the armature coils to the commutator M. Before the test begins, carbon contact rollers 62, 63 and 66 move from the normal positions into contacting positions 62, 63' and i3 3. Rollers 62, 53 (Fig. 25) are pivoted on nuts 62a, 63a, respectively, attached by screws 52a, 53a, respectively, to arms 52?) and 63?), respectively, pivotally supported by screws 62c and $30 respectively (Fig. 26), attached to metal plates 62d and 63d, respectively, which are attached by screws 65 to a carriage 68 and insulated therefrom by a non-conducting plate 86 having ribs 6? (Fig. 31). Carriage 68 has rollersSQ guided by bars it (Fig. 30) and a plate H attached to bars H3 by screws 12. Screws l3 (Figs. 4 and 5) attach plate It to plate H5. The carriage 68 is urged right (Figs. 4, 5 and 27) by springs i i, (Figs. 27 and 28) each attached at one end to a plate i5 which screws I6 attach to carriage 68 and at the other end to a clip TI which screws '58 attach to the carriage The 58. When permitted to do so, springs T l urge the carriage toward the right or down in Fig. 8 to cause the rollers 62 and 23 to engage the cornmutator 65 and to be spread apart so as to move in a position E2 and 53'. This spreading of the rollers is resisted by springs 52c and 636, respectively (Fig. 28), connecting the arms 52?) and 4 insulator 82 attached to plate I5. Lever is pivotally connected with a screw 83 (Figs. 7 and 8) threadedly connected with a sleeve 84 which is threadedly connected with a rod 85 which passe through five non-conducting washers, two of which numbered 86, are at the ends of the group, washers 8? next to the ends and an intermediate washer 88 supported by an arm 89 which as shown in Fig. 25 is integral with or attached to the carriage 68. The rod 85 is surrounded by springs 58 confined between the end washers 85 (Fig. 5) and nuts 9| threaded on rod 85. The function of springs 90 is normally to center the rod 85 relative to the arm 89.

Movementof the carriage 68 toward the commutator causes the roller 64 to move into position 6 concurrently with movement of rollers 62 and 63 into engagement with the commutator. The rollers 62, B3 and 65 are normally held out of engagement with the commutator by the engagement of left end surface 93 (Figs. 4 and 26) of a slot 94 with a screw 95 carried by a lever 96 attached to a bracket 93 attached to plate H, (see also Fig. 17). Lever 95 carries a screw 99 engageable with a bar 520 which is guided for vertical movement in grooves provided by bracket 98 and is retained by plates IflI secured by screws 52 to bracket 93. At its lower end, the bar I50 carries a pin I03 supporting a roller I04 normally received by a recess I08b in the lobe IOBa of a cam plate I08. In the normal position of cam I253, shown in Fig. 17, the carriage 68 is caused to be retained in the position shown in Figs. 4 and 5 against the action of springs "I4 (Fig. 28) so that the rollers 62, 63 are normally held out of engagement with the commutator 60. Referring to Fig. 23, screws 69 secure to cam Hit, cams I95, I22 and I6? and screws IIB secure cam I28 to a hub III which is journaled on a clutch member H2 attached to shaft 2'! and located between cam I88 and a disc IIIa which screws Iilb'attach to hub III. 1y wider than the member I22. Member H2 has clutch teeth 5 23 which are adapted to be engaged by a clutch do IIi (Fig. .21), which an elastic bani H5 urges 'o ard the lute member H2. The H2 carrie a rod or pin IIS adapted to be engaged by a clutch tilI'Oi-'.-*-Ollt cam I I I provided by a bar H8 hie is guided for vertical increment by a liracf et H5 (Fi'z 21) attached to plate the bar being retained by plates I20 and. iii secured to the bracket by he screws I22 and E23 r spectively. H8 extends through a hole in plate I5 and carries a handle or By pushing the knob I25 down t e Far 5 is moves down until the stop pin I28 strikes the bracket I82; and, during this movement, the clutch. throw-out cam Iii is moved a ay from the path of 3 ove ent of the pin H5 and dog H5 can move right (Fig. 21) under the action of band H5 in order to conn ct the shaft 2'! with the drum Iii and the cams I55, I66, I57, I88 connectef. therewith. Normally the dog is held out of eu' agement with the clutch member M2 by a spring confined between a portion of the bracket I59 and the pin E28 attached to the .bar I52. Spring i21 pushes the bar upwardly until a scre'c. I29. carried thereby strikes the plate 'IZI.

An instant after the knob 725 is pushed down the drum ii I starts rotating to move the pin IIS (Fig. an away from the cam III. Then the knob S25 can be released so that the cam Ii? will be returned to its clutch throw-out position so that the dru Iii wiil maize one revolution. Daringthis revoluticncam 52 makes onerevolu- Huh I i I is slight-.

tion to permit the carriage to move toward the commutator so that the rollers 60, 62 and 64 will engage it. Coincidentally with the engagement of cam III with the pin H6 to disengage the clutch, the cam I08 has arrived in such position as to cause the carriage to be retracted and to present the groove I 08b for engagement by the roller I04 (Fig. 17). In order that the cam I08 will not be moved past its position a brake is provided. This brake comprises a shoe I30 (Fig. 4) attached to a bar I3I providing a hole for receiving a stud I32 threaded into the bracket H9 and surrounded by a spring I33 confined between the bar I3I and a nut I34 threaded on the stud. Near its upper end bar I3I has a hole which receives a pin I35 which, as shown in Fig. 6, is attached to bracket II9. During the test the cams I 05-I08 make one revolution and then stop and during that time the tested armature is rotated sufficiently for the required test.

The testing apparatus includes switches S3, S2 and SI which are operated respectively by cams I05, I06 and I! (Figs. 13, 19 and 20 respectively). Cam I01 (Fig. 20) operates also a switch S9. Each of these switches has a roller I40 supported by blade MI for operating a switch contact actuator. The switches SI, S2, S3 and S9 are supported by a plate I44 (Figs. 4, 6) attached to bearing bracket 28. A switch S8 (Figs. 4, 17) supported by a plate I45 attached to bracket 98 has its operating roller I40 engageable with the lug I00a provided by bar I00. Switch S8 is normally closed all during the test cycle and switches SI, S2, S3, S9 are normally open switches closed at different times during the test cycle.

Before starting to test, switch S4 (Fig. 32) is closed to connect a current source (at 110 v. A. C. for example) with wires 20I and 202. Lamp LI shows green to show that S4 has been closed. If switch S8 is closed as it should be when the test cycle starts, lamp L3 shows yellow. At the end of the test cycle, the lamp L3 goes out to indicate the test cycle has ended.

To test an armature after it is connected with the shaft 36, the knob I25 is pressed down to start the cams rotating and the rollers 62, 63 and 64 are advanced into engagement with the commutator 50 and then switch SI is closed which connects coil 50 of switch S with wires 20I and 202. Switch S5 then bridges its contacts 5a and 5b, thereby connecting primary coil I p of transformer TI with wires MI and 202 and connecting secondary coil Is by wires 4:1: and 4 with roller 64. This applies 50 volts to the commutator segments which are connected together by the armature coils. The application of this voltage causes the burning out of any metal particles which might connect the bars of the commutator with the metal core of the commutator which is grounded through the apparatus.

Then switch SI opens to effect opening of switch S5 and switch S2 closes to connect coil 60 of switch S6 with wires 20I and 202 which causes switch S6 to bridge its contacts 6a and 6b, thereby connecting primary coil 2p of transformer T2 with wires 20I and 202 and connecting secondary coil 2s of transformer T2 with contact roller 64 through wires 49: and 4 and connecting coil 28 with ground through coil relay REIc and resistance RIO with ground. This causes the application of 500 volts to the commutator. If the commutator is not grounded coil REIc is not energized and relayREI does not closer If the commutator is grounded, contacts I second. One end of coil WC is connected with" REIa of relay REI close. Coil RE2c of relay RE2 is energized through the following circuit:

Wire 20I, normally closed switch S8 (remaining closed until near the end of the cycle), switch S9 (normally open but closed by cam I07 near the end of the ground test), contacts REIa of relay REI, wire 206, coil REZC of relay RE2 and wire 202. Relay RE2 closes its contacts REZa and E5122); and when, contacts RE2b are closed, RE2c remains energized through the following circuit independent of contacts REIa of relay REI, wire 20I, switch S8, wire 203, contacts REZb, coil RE2c, wire 202. When contacts REZa closered lamp I2 burns through the following circuit: Wire 20I, switch S8, wire 203, contacts REZa, wire 205, lamp I2, wire 202. Lamp I2 burns red to indicate ground and remains closed until switch S8 opens near the end of the cycle. In case there is no ground, lamp I2 does not burn.

The last event in the cycle is known as the high frequency test. Before describing that test, the high frequency apparatus will be described. There is a power pack including transformers T3, T4, T5 and T6 whose respective primary windings 3p, 4p, 5p, 6p are connected with wires 20I and 202. Secondary 33 of transformer T3 is center-tapped to ground and its ends are connected with the plates of rectifier tubes V3 and V4 whose cathodes are heated by current furnished by secondary winding 43 of transformer T4. The rectified current from tubes V3 and V4 is filtered by network comprising chokes HI and H2 and condensers C6 and C1; and the filtered current passes along wire 2II to variable voltage divider resistance R2 and to fixed divider resistances R5 and R6.

Wire 203 connects resistance R2 with grid 2 I2 of tube VI. Wire 204 connects resistance R2 with resistances R3 and R4 connected respectively with grid 22I of tube V2 and with condenser C9 also connected with plate 220 of tube V2. One terminal condenser CIO is connected with ground of resistance R6 and its other terminal is connected with the junction of resistances R5 and R6 and with wire 205 connected with the cathode 223 of tube V2. The cathode heater 224 of this tube is connected through connections y, 1 with the similarly marked terminals of secondary winding 5s of transformer T5. The secondary winding 6s of transformer T6 is center tapped to ground and is connected as indicated by m, :0 with cathode 2I4 of tube VI. Grid 2I3 of tube VI is connected with ground through condenser C5 and is also connected with a grid coil GC connected with adjustable resistance RI which is grounded and has its junction with coil GC connected with one terminal of condenser C4 and whose other terminal is connected with ground and also with a wiper 2I5 adjustable along resistance RI. Grid 2II of tube VI is grounded. Plate 2I0 of tube VI is connected by Wire 2I6 with terminals of condenser C2 and a plate coil PC whose other terminals are connected with wire 2I'I which is connected with the power pack. Wire 2 I! is also connected with grounded condenser C3.

The network which includes tube VI and coils GC and PC provide what is generally known as an Armstrong oscillator. Coils PC and GC and a work coil WC are inductively related by concentric location. The oscillator is adjusted for a suitable frequency, for example 5 kc. per

the grounded end of condenser CI which wire 3 connects with the other terminal coil WC and with a meter M connected by Wire A l with a terminal of condenser C9 whose other terminal is connected with plate 220 of tube V2.

The high frequency test is started by the closing of switch S3 which connects coil 1c of relay switch S1 with wires 2M and 202 to effect the bridging of pairs of contacts 20, 4c and 30 for the purpose of completing the'circuits between the commutator engaging rollers and the test circuit. A portion of the test circuit is shown in Fig. 33 in simplified form. The input circuit may -be considered to include the grounded end of coil WC, wire 3, contact roller 63, the contacted commutator bars, armature coils ac between rollers 03 and 04, roller 64, wire 0 and its ground connection with the coil WC. The output or meter circuit includes contact roller 64, wire 4 meter M, condenser C9, tube V2, condenser CIO, ground to resistance R1, condenser C8, wire 2 and contact roller 02 and armature coils ac between rollers 62, 64. The reading of the meter M is under control by tube V2 whose grid 222 is connected with wiper 225 of resistance R1. If the armature is good, that is, it has no open circuited coils, short-circuited coils and no grounded coils, it combines with work coil WC and condenser CI to provide a circuit which has a resonance approaching full resonance with respect to the oscillator. In Fig. 34, curve RR represents relative resonance. If the armature is good, point A on curves RR represent the relative resonance of the armature and work coil circuit to the oscillator. Point A is well up on the gradual scope of the resonance curve but is somewhat below the maximum. If the armature is good the meter reading is of a substantial constant even though the armature is rotating slowly and adjustment of resistance RH can be made so that the meter will read, for example, as represented by line A in Fig. 35, 90 units. If the armature has one or more open circuited coils, the relative resonance will increase to B on curve RR; and the meter needle will fluctuate between 90 units and an increased reading of 110, for example, as represented by line B in Fig. 35. If the armature has shorted turns, its relative resonance will be decreased to a value represented by point C on curve RR; and the meter needle will fluctuate between 90 units and a, lower value such as '70 units, for example, as represented by line C, Fig. 35. If the armature is grounded, the relative resonance will be decreased to a much lower value such as represented by point D on curve RR; and the meter will read a much lower value, for example 30 units, as represented by line D in Fig. 35.

The voltage impresed on grid 222 of tube V2 is under control by the resonance of the work coil circuit relative to the oscillator circuit. If the relative resonance is greater than normal for a good armature, such as value B; grid 222 is so biased as to permit tube V2 to pass more current hence the meter M will read higher. When the relative resonance is less than normal grid 222 is so biased as to permit tube V2 to pass less current and the reading of meter M will be lower.

The high frequency test provides for detection of good, open-circuited, shorted or grounded armature by the use of resonance comparing means comprising a first circuit including a condenser and a coil subjected to high frequency and a second or armature circuit including armature coils, a second condenser and a second coil connected therewith and inductively coupled with the coil of the first circuit and a device for indicating the degree of resonance of armature circuit with respect to the first circuit. A meter capable of reading from 0 to 1 milliamperes, when used with an amplifier, is a suitable indi-- cating device. Its scale should be read from 0 to 120 units.

Other suitable electrical values are as follows:

Condensers C1, C2, C3- 2 mfd., 1000 volts Condenser C4 1 mid, 600 volts Condenser C5 .02 mfd., 600 volts Condensers C6, C1, C8. 2 mid, 1000 volts Condenser C9 1 mfd., 600 volts Condenser C10 50 mfd.,.l50 volts 7 Resistance R1 25,000 ohms, 50 watts Resistance R2 5000 ohms, '200 watts Resistance R3 10,000 ohms, 10 Watts Resistance R4 1. 10,000 ohms, 600 watts Resistance R5 50,000 ohms, 100 Watts Resistance Re 750 ohms, 20 watts Resistances R1, R8". 250,000 ohms, 1 watt Chokes H1, H2 10 henrys, 200 milliamps. Transformer T1 50 volts, 1000 v. a. Transformer T2 500 volts, 5 v. a. Transformer T3 1100 volts, 200 ma. center tap Transformer T4 2.5 volts, 5 amp. Transformer T5 6.3 volts, 3 amp. Transformer Te 10.0 volts, 10 amp. centerv tap Tube V1 #813 Tube V2 #GLG Tubes V3, V4 #816 Fig. 36 shows the timing of the apparatus relative to motion of cams I05, I06, I01, I08 for one revolution. The zero positions of these cams are shown in Figs. 17 to 20. At about 7, the rollers 62, 03, 64 begin to advance to the commutator. From 15 to the commutator is subjected to burn out voltage. From to the armature is subjected to the ground test. From to about 360 the armature is subjected to the high frequency test. During one revolution of the cams, the armature rotates about three times at a rate of about 10 R. P. M.

While the embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. Apparatus for testing an armature having coils connected with bars of a commutator, means for rotating the armature, three, spaced, commutator engagin contacts, means for causing the contacts to engage the commutator while rotating, an oscillator of fixed frequency, an input circuit including a resonator inductively related to the oscillatorand two of the contacts between which armature coils are'connected and an output circuit including one of the contacts in the input circuit and another of said contacts between which armature coils are connected, and including a meter giving, as the armature rotates, an indication of the resonance of thecombination of coils ofthe armature and said resonator with respect to the resonance of the oscillator, said indication being in accordance with Whether armature coils are all in proper condition or whether one or more of them are open circuited, short-circuited or grounded.

2. Apparatus for testing an armature having coils connected with bars of a commutator and supported by a core and a shaft supporting the core and commutator, said apparatus comprising a. chuck for receiving and rotating the shaft and providing a connection to ground, means for rtating the armature, three spaced, commutator engaging contacts, means for causing the contacts to engage the commutator while rotating, an oscillator, an input circuit connected with ground and including two of the contacts between which armature coils are connected and a resonator inductively related to the oscillator of fixed high frequency, and an output circuit connected with ground and including one of said contacts in the input circuit and another of said contacts between which armature coils are connected, and including a meter giving, as the armature rotates, an indication of the resonance of the combination of coils of the armature and said resonator with respect to the resonance of the oscillator, said indication being in accordance with whether armature coils are all in proper condition, or whether one or more of the coils are open circuited, short-circuited or grounded.

3. Apparatus for testing an armature having coils connected with bars or a commutator and supported by a core and shaft supporting the core and commutator, said apparatus comprising a chuck for receiving and rotating the shaft and providing a connection to ground, means for rotating the armature, three spaced, commutator, engaging contacts, means for causing the contacts to engage the commutator while rotating, an oscillator of fixed high frequency, a resonator comprising a work coil inductively related to the oscillator and a condenser in parallel with the work coil, an input circuit having connection with ground and comprising in series the resonator and two of the contacts which have armature coils connected between them and an output circuit connected with ground and comprising, in series, one of the contacts in the input circuit and another of the contacts, said contacts having armature coils connected between them, an ammeter, a thermionic tube, a condenser and a .variable resistance, said tube having its grid con nected with said resistance, said meter giving as the armature turns an indication of the resonance of the combination of coils of the armature with the work coil and parallel condenser relative to the resonance of the oscillator, said indication being in accordance with whether armature coils are all in proper condition, or whether one or more of the coils are open circuited, short-circuited or grounded.

FLOYD J. FOUST.

OLVEN W. CHILDRESS, JR.

DWIGHT O. CRIM.

ROBERT A. THOMPSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,253,211 Chubb Jan. 15, 1918 1,276,304 Zeek Aug. 20, 1918 1,432,003 West Oct. 17, 1923 1,479,284 Cullin Jan. 1, 1924 1,764,709 Anderson June 17, 1930 1,906,030 Whelchel Apr. 25, 1933 2,108,637 Bartgis Feb. 15, 1938 2,472,018 Johnson May 31, 1949 

